The background art of the present invention will be described by taking a hydraulic shovel shown in FIG. 5 as an example.
This hydraulic shovel comprises a crawler-type lower propelling body 1, an upper slewing body 2 mounted on the lower propelling body 1 slewably about an axis X extending in a direction perpendicular to the ground, and a working attachment (excavation attachment) 9 attached to the upper slewing body 2, wherein the working attachment 9 includes a boom 3, an arm 4, a bucket 5, and a plurality of hydraulic actuators for operating them, that is, a boom cylinder 6, an arm cylinder 7 and a bucket cylinder 8. The hydraulic shovel further comprises, as other hydraulic actuators, right and left travel motors for driving the lower propelling body 1 (right and left crawlers), and a slewing motor for slewing the upper slewing body 2.
In this type of hydraulic shovel, there can be required that the slewing movement of the upper slewing body 2 by the slewing motor and other movements by the hydraulic actuators other than the slewing motor is performed independently from each other. As means to ensure the independence, the following Patent Document 1 discloses: dividing a hydraulic circuit to be equipped in the hydraulic shovel into (i) a first circuit to which one of the right and left travel motors and the boom cylinder belong, (ii) a second circuit to which the other travel motor and the arm cylinder belong, and (iii) a third circuit to which the slewing motor belongs; and providing first, second and third pumps for driving the first, second and third circuits in a mutually independent manner.
Besides, this hydraulic circuit is further designed to ensure straight-traveling stability. Firstly, the travel motors included in respective first and second circuits are located on respective upstreammost sides with respect to respective flows of hydraulic fluid discharged from the first and second circuits, so that each of the first and second circuits is set as a circuit for prioritizing travelling. In other words, the first and second circuits are configured so as to give each of the travel motors a priority to be supplied with hydraulic fluid discharged from each of the first and second pumps during a double travel operation for simultaneously driving the two travel motors. Secondly, the hydraulic circuit includes a merging valve for merging hydraulic fluid discharged from the third pump toward the third circuit into a fluid passage leading to the actuator other than the travel motor, in each of the first and second circuits. This hydraulic circuit makes it possible to ensure a movement of each of the remaining hydraulic actuators other than the travel motors, while guaranteeing straight-traveling stability.
The hydraulic circuit includes a plurality of control valves for operating respective hydraulic actuators, wherein each of the control valves has a bleed-off passage. Thus, when the arm cylinder in the second circuit is not operated, i.e., when the control valve for operating the arm cylinder is in a neutral position thereof, the bleed-off passage of this control valve is communicated with a tank to thereby allow hydraulic fluid discharged from the third pump to flow into the tank via the second circuit, thus preventing a pumping pressure of the third pump from sufficient rise. This slows down the movement of each of the remaining hydraulic actuators such as a boom raising movement or a slewing movement. This, however, causes no problem because the above movements are to be performed during traveling; it is rather desirable in view of safety.
In the above conventional hydraulic circuit, the merging valve is adapted to merge hydraulic fluid discharged from the third pump into each of the first and second circuits, regardless of presence or absence of an operation on the two travel motors, and even also when a boom raising operation is performed, in the same manner as that when operations on the two travel motors and at least one of the remaining hydraulic actuators are performed. The conventional hydraulic circuit therefore has a problem that a sufficient pumping pressure cannot be obtained during the boom raising operation irrelevant to the travel operation, resulting in poor performance of the boom raising movement. Specifically, upon the boom raising operation with no operation of the arm cylinder in the second circuit, wherein the control valve for the arm cylinder is in the neutral position to communicate the bleed-off passage of this control valve with the tank, hydraulic fluid discharged from the third pump flows into the tank through the second circuit, thereby hindering the pumping pressure from sufficient rise. As measures against this problem, there can be performed providing an orifice in a merging passage extending from the third circuit to the second circuit to increase the pumping pressure; however, there are remarkable limitations on a rise of the pumping pressure by means of the orifice.
Besides, the conventional hydraulic circuit has a defect that the performance of both of the boom raising operation and the slewing operation deteriorates the slewing acceleration performance due to difficulty in raising pressure for slewing acceleration, in addition to the boom raising performance. Thus, there can be a problem that a cycle time of the work, for example, of loading the bucket with earth and sand and dumping it is extended to thereby deteriorate the work efficiency.
The conventional hydraulic circuit may be designed such that hydraulic fluid discharged from the third pump is merged into the first circuit via a path different from the merging valve, during the boom raising operation; however, such a design is inadvisable, because it involves an increase in complexity of a connection portion between the circuits of the first to third circuits and a need for a special valve, resulting in increased complexity of circuit configuration and increased cost.